1. Field of the Invention
The present invention relates to a method of thermal printing by the deposition of dyes.
2. Description of the Related Art
The present invention relates more particularly to a method of continuous-tone dye diffusion printing of the type described in the articles by P. W. Webb and R. A. Hann, &lt;&lt;Measurement of thermal transients in a thermal print head used for dye diffusion color printing&gt;&gt; in IEEE Proceedings-A Vol. 138, No. 1, January 1991, and A. Kaneko, &lt;&lt;A Simple Simulation for Simultaneous Diffusion of Dye and Heat in Dye Diffusion Thermal Transfer Printing&gt;&gt;, in Journal of Imaging Science, volume 35, No. 4, July/August 1991.
A method of this kind, which can be used to achieve high-quality printing, can be applied in particular to the customization of plastic cards such as smart cards, magnetic cards, badges, etc.
FIG. 1 shows a printing device 1 according to this method, designed for the customizing of plastic cards of a known kind as already described in the French patent applications No. 90 14329 or No. 94 02116 filed on behalf of the present Applicant.
In very broad terms, the printing device 1 comprises two pairs 2, 3 of secondary rollers for the conveyance of a plastic card 4 to be printed, a main conveyance and printing roller 5, a print head 6 of which only the useful bar-shaped end is shown, an inking ribbon 7 with three sequences of dyes of primary colors, generally yellow, (J), magenta (M) and cyan blue (C). The card 4 is sandwiched between the print head 6 and the main roller 5 with the interposition of the inking ribbon 7. The card 4 moves step by step in a printing direction S identified in FIG. 1 and, to each shift of the card, there corresponds an equivalent shift of the inking ribbon 7 and the printing of a line. Thus, the printing of a pattern proceeds line by line for a first primary color sequence until the entire length of the card is crossed, then the card returns to the initial position for the printing of a second primary color sequence etc. After three printing sequences, a full range of colors is obtained by the combination of the three primary colors.
FIG. 2 shows the lower face of the print head 6 in contact with the ribbon 7, and FIG. 3 gives a schematic view of the electrical structure of the print head 6. Together, these two figures provide for a clearer understanding of the printing mechanism.
As can be seen FIG. 2, the print head 6 includes a row of n heating resistive points P.sub.i (P.sub.1, P2, . . . P.sub.n), i being a index ranging from 1 to n. For the printing of a line, each resistive point P.sub.i is activated by a train of voltage pulses of the same duration, and is thus brought up to a temperature of diffusion of the dye coating the ribbon 7, namely a temperature of about 200.degree. C. to 300.degree. C. Each resistive point P.sub.i thus ensures the printing of a pixel, the set of pixels constituting a line. Of course, when an pixel should not be printed, the corresponding resistive point P.sub.i is not activated.
In FIG. 3, it can be seen schematically that the constant-duration voltage pulses providing for the activation of the resistive points P.sub.i are applied by means of a plurality of switches I.sub.i (I.sub.1, I.sub.2, . . . I.sub.n) connected to a source 8 of voltage Va by means of an electrical cable 9. The switches I.sub.i are controlled by an electronic circuit 11 that opens and closes them alternately. Since the quantity of dye deposited on the card by diffusion (the term &lt;&lt;migration&gt;&gt; is also used) is a function of the temperature of the resistive points P.sub.i, the electronic circuit 11, depending on the image to be printed, determines the number of voltage Va pulses that should be applied to each resistive point P.sub.i. The quantity of primary color deposited for each pixel is thus modulated, making it possible to obtain a large variety of shades of colors after the combination of the three primary colors.
In view of the above, it can be understood that, to print a pattern with a constant intensity of color, it suffices in principle to apply the same number of electrical pulses to the resistive points P.sub.i concerned, at each printing of a line. However, in practice, this result is not obtained and variations in color intensity occur depending on the shape of the printed pattern. For example if, as shown in FIG. 4, a gradually widening strip is printed on a card 4, then it is observed that as the strip widens, the color deposited becomes lighter. In general, it can be seen that the intensity of the color becomes smaller as the width of the printed pattern increases.
Such variations in color intensity originate in an problem that is electrical in nature. More specifically, when the printing of a line requires that a large number of resistive points P.sub.i should be activated at the same time (for large-sized patterns), a major current draw takes place in the voltage source 8 and the voltage Va provided to the print head 6 decreases appreciably. A voltage drop of this kind is due to various electrical losses by Joule effect between the source 8 and the print head 6, especially in the cable 9 which has a considerable length because of practical imperatives. Conversely, when the printing of a line requires the activation of only a small number of resistive points (for a small-sized pattern), the current is weak and the voltage drop is negligible.
To mitigate this drawback, there has already been proposed a thermal printing method using a print head that comprises a plurality of resistive points activated by pulses of a supply voltage liable to fluctuate as a function of the number of simultaneously activated resistive points, in which the activation of the resistive points is controlled by a control signal whose duration is determined so that the energy provided to the resistive points by each of the voltage pulses is appreciably constant and independent of the fluctuations of the supply voltage. A method of this kind is described by the U.S. Pat. No. 4,434,354.
However, for its implementation, this prior art method requires the designing of a relatively complex switch-over circuit that is sensitive to the supply voltage and determines the instant at which the activation pulse must be stopped. This switch-over circuit made out of analog components is difficult to implement, proves to have low precision in use and has a considerable cost price.
Again, the document PATENT ABSTRACT OF JAPAN, vol. 9, No. 322 (M-440) [2045] Dec. 12, 1985 & JP-A-60 155 475 Aug. 8, 1985 describes a method for the control of the duration of a signal to activate resistive points of a print head wherein:
1) the activation device is delivered by a timing mechanism, PA1 2) the supply voltage is sampled at a predetermined instant after the activation signal has been sent out, PA1 3) the desired duration of the activation signal is then computed, PA1 4) the desired duration of the activation signal, once computed, is sent to the timing mechanism which stops working when this duration is reached.
Thus, one drawback of the prior art methods is that they are complex to implement.